1. Field of Utilization
The present invention relates to a process for sterilization, particularly with ultraviolet ray, which is utilized in the technological field of sterilization for vacuum-packaged fresh food such as raw meat i.e. lump or dressed meat of beef, pork, chicken, lamb and the like.
2. Prior Art
It is known that the irradiation of ultraviolet ray with a wave length of 235-275 nm on surface of food has a sterilizing effect.
Utilizing this effect, the irradiation of ultraviolet ray on food surface and the sterilization of food with ultraviolet ray after its vacuum-packaging have been also introduced in the field of food except raw meat. Nevertheless, no example for application of the technique on raw meat has been found.
Conventionally, raw meat such as chilled beef is used to be preserved in tight conditions caused by heat shrinkage of film after the meat is vacuum-packaged with heat shrinking multi-layer film having a barrier function. Certain bacteria in no small quantities are likely to adhere to the surface of packaged raw meat in a period preceding packaging process and owing to their growth, the number of bacteria increases as a preservation period after packaging extends. The extension of the preservation period is desirable while keeping the number of bacteria within the permissible range. It depends on the initial number of bacteria in a fixed condition for preservation.
The combination effect of ultraviolet ray and heat has been described also in the sterilizing method by direct irradiation of ultraviolet ray on food after heating process [Japan Food Industry Society, The 29th Assembly (1982)].
The synergistic effect between heat and ultraviolet ray has been known as described previously. Hence the inventors have found that the ultraviolet sterilization incorporated into the conventional packaging process for chilled beef i.e. the process comprising packing, vacuum-packaging, heat shrinkage and water cooling, is capable of doing the following;
1. to decrease the number of bacteria adhered on the surface, PA1 2. to inhibit the increase of bacterial growth in number during the preservation period, and PA1 3. to depress the discharge of acidification smell and gas. As a result, the inventors have achieved the present invention.
It has been also found that the ultraviolet sterilization process may be significantly carried out when it is incorporated after vacuum-packaging and before heat shrinkage and in that case, the effects achieved by the process may be remarkable.
In relation to the combination effect of ultraviolet ray and heat, the process comprising vacuum-packaging, ultraviolet sterilization and heat shrinkage in sequence according to the invention may have much greater effects than that comprising vacuum-packaging, heat shrinkage and ultraviolet sterilization in sequence because in the latter process the following is considered to occur.
1. Heat melts fat on surface of raw meat more or less, so that the bacteria present on the surface of fatty portions and meat portions adjacent to the fatty portions are covered with the melted fat. As a result, the irradiation of ultraviolet ray thereafter can not sufficiently reach the bacteria.
2. Heating causes the tissue on raw meat surface to get loose so that meat juice extrudes into the surface thereof to inhibit the transmission of ultraviolet ray.
3. Raw meat is not of regular shape and thus, hot water is likely to remain in sunken portions on the surface of film packaging in the case of hot water heating, so that the transmissivity of ultraviolet ray reduces. Moreover, the hot water remaining acts as lenses so that ultraviolet ray does not reach the surface of raw meat and the like uniformly.
4. Ultraviolet irradiation after heating lengthens the period from heating to cooling because the process of ultraviolet irradiation requires some time, whereas some change of products such as in color is liable to occur to make their values as merchandise lower.
5. In the case of heating by hot water and steam, packaging material changes its color into white because of absorption of water so that the transmissivity of ultraviolet ray reduces.
6. In the case of heating by hot water and steam, water drop is likely to adhere to the surface of the packaging material and the drop falls down, causing contamination of ultraviolet lamps underneath; evaporation of water drop also causes contamination of ultraviolet lamps in surrounding environment, resulting in gradual reduction of irradiation dose of ultraviolet ray. For its prevention, frequent cleaning is required and it takes a lot of time and labor.